1. Field of the Invention
The present invention relates to an arthroscopic indenter in general and, more particularly, to an apparatus and method for measuring internal properties of cartilage.
2. Description of the Relevant Art
The effects of articular cartilage degenerative diseases (such as osteoarthritis or chondromalacia patella) are visible to the naked eye when the disease has reached an advanced stage. Manifestations of these diseases include changes in the tissue's: 1) biomechanical properties (stiffness, permeability, compressibility), 2) biochemical composition (type II collagen, proteoglycan macromolecules, interstitial water content), and 3) morphological characteristics (surface fibrillation and fraying, osteophyte formation). At the early stages of cartilage degeneration, the tissues's stiffness decreases and its compressibility and permeability increase. Thus, a reliable means to quantify the initial stages of cartilage degeneration is to obtain its mechanical (or material) properties. This can be accomplished during arthroscopy, which is an in vivo and in situ procedure, using a probe to examine qualitatively the articular surfaces. Using direct vision provided by an arthroscopic fiberoptic tube connected to a videocamera, the probe is used to palpate the tissue and, based on the tissue's indentation, the orthopaedist decides on the existence or severity of the disease. During this procedure, the orthopaedist also examines visually the surface characteristics of cartilage. This procedure is neither objective nor successful in determining the early stages of degenerative diseases, during which visual abnormalities are not present.
A device used to measure the "deformation resistance" of tissue, and particularly the articular surface of the patella, is described in U.S. Pat. No. 4,364,399. This arthroscopic instrument simply measures the amount of resistance pressure exerted by the cartilage at a given indentation. Positioning of the probe is manually accomplished, and perpendicularity of the probe relative to the cartilage surface is subjectively determined. The distance of indentation is mechanically calculated often using manual placement of the pressure transducer against the cartilage surface. Manual indentation process is not sufficiently accurate to allow repeatable, objective measurements. Manual indentation devices cannot programmably vary the applied indentations or forces in order to more accurately obtain mechanical characteristics of the cartilage. This device does not measure the thickness of articular cartilage. Two tissues with the same mechanical properties but unequal thicknesses will exhibit different deformation or force resistance. Thus, the thickness of the tissue must also be measured and used to normalize the measured tissue deformation or force resistance. Furthermore, the device of U.S. Pat. No. 4,364,399 is used to apply indentations onto cartilage without immobilizing the cartilage's subchondral bone relative to the device. Thus, under indentation, not only cartilage but other surrounding or underlying soft tissues deform. As a result, when both the cartilage and surrounding tissue deform, the applied cartilage indentation is not accurately known, and the measured cartilage resistance may be irrelevant. Thus, manual indentation devices provide an extremely subjective value of the tissue's deformation and force behaviors.
While indentation techniques are preferable over arthroscopic observations, manual indentation techniques do not provide sufficient data to allow accurate and repeatable internal mechanical measurements to be taken of the cartilage.